This invention relates to an automatic takeup and overload protection device for shape memory metal actuators.
Shape memory metal actuators are comprised of materials characterized by their ability to deform in shape at one temperature and then revert to their original shape when the original temperature is restored. Shape memory metal (SMM) actuators utilize these materials to mechanically operate a mechanism. Typically the SMM element is in the form of a wire, ribbon, or spring and is composed of a material such as Nitinol which contracts upon heating and returns to its original length upon cooling. Because the SMM element is of finite strength, such actuators may require some form of overload protection to prevent excessive stresses in the SMM element.
In the past, such overload protection has been provided by a biasing element, typically a coil or leaf spring, which is mechanically connected in series to one end of the SMM element. The spring constant of the overload protection spring is selected such that the force required to expand or compress the spring is greater than the force required to operate the actuator. Therefore, the overload spring is operative only when the load on the SMM element exceeds the normal operating range of the actuator.
A disadvantage of using SMM material in actuators is that the SMM element tends to distort in the direction of the load as the actuator is cycled, a phenomenon commonly known as cyclic creep. Cyclic creep often results in an extra length in the SMM element, both when the element is hot and when it is cold. Upon heating, the SMM element must take up this slack before the element can perform its function of moving the attached load. When the SMM element becomes so long that it can no longer move the load through its full excursion, the SMM actuator must be adjusted or replaced.
It is an object of this invention to provide an overload protector for the SMM element which simultaneously compensates for any increase in length of the SMM element caused by cyclic creep.